As an apparatus for recording a color image that has been arranged to form the record dots of at least three colors on a recording medium, there is known a melt type color thermal transfer recording apparatus. In the melt type color thermal transfer recording apparatus, there is used an ink sheet on which there are longitudinally coated heat-meltable inks that have respectively three colors of yellow ("Y" hereinafter), magenta ("M" hereinafter), cyan ("C" hereinafter) or four colors having these colors and a color of black ("K" hereinafter) added thereto in this order. And, this ink sheet and a transfer paper are superposed one over the other and heat is applied from a thermal head to the ink sheet to thereby melt the ink and thereby transfer "Y", "M" and "C", or "Y", "M", "C" and "K", to the transfer paper in this order. The thermal head is equipped with a large number of heat generating elements that have been disposed in the form of a line in the main scan direction. By controlling the time width in which current is applied to these heat generating elements, it is possible to control the area of the record dot to be transferred onto the transfer paper and thereby express the gradation sequence.
FIG. 19 illustrates an example of the disposition pattern of the record dots that have been formed on the transfer paper through the operation of a conventional melt type color thermal transfer recording apparatus. This pattern is one wherein the record dots of each of four colors of "Y", "M", "C" and "K" are formed at 300 dpi (dot pitch .apprxeq.84.7 .mu.m) in each of the main scan and sub-scan directions. Further, by shifting the record dots in each line every other row in the sub-scan direction by 1/2 (.apprxeq.42.3 .mu.m) of the dot pitch, the heat of the thermal head is diffused to thereby enable the formation of excellent record dots. In this specification, shifting of the record dots in each line every other row in the sub-scan direction in this way is called "one-dot zigzag printing".
FIG. 20 illustrates an example of the timing of pulses which are applied to an energization control switch of the thermal head when realizing the above-mentioned print pattern. It is to be noted that since in this figure for the convenience of not only illustrating the timing for realizing the print pattern such as that illustrated in FIG. 19 but also illustrating the gradation level concurrently, with the gradation in the first row being set to be 63 under the assumption that the maximum gradation level is 63, the gradation levels in the second row and thereafter succeeding rows are made to become gradually lower, it does not result that the areas of the record dots formed by this current correspond to those illustrated in FIG. 19.
As illustrated in FIG. 20, in either the odd row that stands in an odd number of order or the even row that stands in an even number of order, application of current is started in units of 10 msec and this application is stopped after the lapse of a time period that corresponds to the gradation level. And, a time difference of 5.0 msec is provided between the timing for starting the application of current in a line in an odd-order row and the timing for starting the application of current in the line in an even-order row. This timing is determined by being synchronized with a timing pulse that is generated in units of 2.5 msec. It is to be noted that this timing pulse is also synchronized with the timing for driving a stepping motor for conveying the transfer paper. Here, vertical broken lines that have been drawn commonly to all rows in units of 300 dpi/10 msec are reference lines that have been drawn at equal intervals from the print starting timing.
In a case where the resolution as viewed in the sub-scan direction is 300 dpi as illustrated in FIG. 19, it is ideal to dispose the record dots accurately at pitches of 84.7 .mu.m in the sub-scan direction. However, so long as the ink sheet and transfer paper are respectively mechanically conveyed using stepping motors, it is at present extremely difficult to dispose the record dots with the positions thereof being not departed even 1 .mu.m from each other over an entire recording region of every transfer paper. Accordingly, it is actually unavoidable that the departures are made somewhat (several .mu.m to several tens of .mu.m) from the regular positions in the sub-scan direction. In addition, these departures occur at random between each color or between adjacent prints generally with some extent of widths (several mm to several tens of mm) as viewed in the sub-scan direction. FIG. 21 illustrates an example of a disposition pattern of record dots where there exist portions at which "C" departs relatively from "Y" and "M".
In a case where the record dots have been relatively departed in this way, even when each of "Y", "M" and "C" has been printed at a gradation of 50%, the color on the transfer paper does not become a uniform half-tone gray. The reason for this is because by the record dots of a given color being relatively departed from the record dots of another color in the sub-scan direction the hue that is seen with the naked eyes undesirably changes with the result that hue streaks (also called "color moire") occur. Further, the fact that a difference in respect of the transferability (the ink concentration with respect to the energy) exists between where ink is transferred directly onto the transfer paper and where ink is applied onto the ink of another color also promotes the occurrence of the color moire.